Recently, a magnetic tape is increasingly required to meet long recording, and for long recording, it is required to store a longer tape in a tape reel package by decreasing the thickness of the tape. It is necessary to decrease the thickness of the base film for decreasing the total thickness of the tape. In this actual case, however, the stiffness of the tape decreases. As a result, in some cases, the tape edge is liable to be damaged in loading and unloading, or the tape is liable to be deformed to cause a strain in record when a high instantaneous pulling force is exerted on the tape. Therefore, the film for use as a base for a long-recording magnetic recording medium is required to have a high Young's modulus.
Further, with a camera-integrated VTR being widely used, the VTR is used outdoors or kept in an automobile, and the tape is often exposed to severe temperature conditions. Therefore, the tape is increasingly required to have dimensional stability sufficient to cause no skew strain, or ultimately, the base film is strongly required to have sufficient dimensional stability.
A biaxially oriented polyethylene terephthalate film has been conventionally used as a base film for a magnetic tape. A so-called supertensilized film whose Young's modulus in the longitudinal direction is increased for long recording has a Young's modulus of 850 kg/mm.sup.2 at most, and in this case, the maximum Young's modulus in the transverse direction is 450 kg/mm.sup.2 at most. On one hand, when the Young's modulus in the longitudinal direction is increased, the Young's modulus in the transverse direction inevitably decreases. As a result, the tape is liable to be damaged in the edge portion when running. On the other hand, when the Young's modulus in the width (lateral) direction is intended to be increased in the tape production, inevitably, no sufficient Young's modulus in the longitudinal direction can be obtained. As a result, the contact of the tape to a magnetic head comes to be poor, and an output fluctuation would occur. Further, a base film whose Young's modulus is increased by stretching it at a high stretch ratio has a problem in that it has low dimensional stability since there still remains a strain which has occurred in a step of film forming. Further, processing at a high stretch ratio has another problem in that the product yield decreases. Prior art techniques inevitably include a step of removing a residual strain by relaxation heat-treatment or aging treatment in a step of forming a base film or in a step of processing a tape, and involve complicated selection and combination of production conditions.
The field of use in magnetic recording has been recently increasingly requiring the achievement of high-density recording and high-quality recording, and a polyester film which constitutes a base film is increasingly required to be excellent in surface flatness and slipperiness and to be also excellent in running durability and abrasion resistance.
For improving lubricity, there has been conventionally proposed a method in which inert fine particles are incorporated into polyester or a method in which fine particles are allowed to precipitate in a polymerization system during the polyester synthesis. These methods are aimed at improving the lubricity of a film by forming protrusions derived from the fine particles on the film surface at the time when the film of polyester is formed. In the method of improving the slipperiness of a film by means of fine particles as described above, generally, the slipperiness is improved with an increase in the degree of surface roughening. However, on the other hand, the surface to which a magnetic coating composition is applied is roughened due to this surface roughening, and consequently, the electromagnetic conversion characteristic is deteriorated.
For solving the above contradictory flatness and lubricity, a variety of means have been proposed which allow large-sized particles and small-sized particles to be co-present in combination. However, these means also involve problems, and these means find it difficult to comply with the requirement to achieve a high grade, e.g., a high-density and high-quality, of a magnetic recording medium. The reasons therefor are as follows: The size of large-sized particles used in composite inert particles is too large and coarse to meet with the requirement for a quality of a high grade. Further, with an increase in the size of the large-sized particles, protrusions on the film surface have a greater height, at the same time the size of voids around particles increases, so that the high protrusions are abraded off in a calendering process. As a result, drop-out is caused. In addition, since it is difficult to control the distribution of the fine particle, it is difficult to adjust the protrusions on the film surface as designed.